Protective arrangement for rectifiers



Sept. 5, 1944. c. w. HANSELL PROTECTIVE ARRANGEMENT FOR RECTIFIERS 2 Sheets-Sheet 1 Filed April 24, 1941 all!!! INVENTOR we?! MW ATTORNEY Sept. 5, 1944. c, w, H L 2,357,401

- PROTECTIVE ARRA GEMENT FOR RECTIFIERS Filfid April 24, 1941 2 Sheets-Skeet 2 INVENTOR 6 Patented Sept. 5, 1944 UNITED STATES PATENT OFFICE Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 24, 1941, Serial No. 390,040

This invention relates to a new and useful rec- 4 Claims.

tifier protective system.

An object of this invention is to provide a rectifier system which will suppress flash-backs or overloads occurring in the system.

Another object of this invention is to improve the operation of a radio transmitter by the use of non-linear resistors.

A feature of my invention is that, in rectifier protective systems, I make use of the overload current, itself, to provide energy'to stop itself and to overcome the inherent disadvantages of previously known rectifier systems of which I am aware.

In the prior art, wherein grid-controlled gaseous discharge rectifiers are used, it is common practice to stop flash-backs or overloads by applying a negative blocking potential to the rectifier grids. Although these arrangements are in common use, they are subject, nevertheless, tov

a fundamental difficulty which can often cause failure of the system to function as intended.

For example, in one system there was provided a has certain objectionable features, particularly when applied to circuits for radio transmission, primarily because of the time required for the relays to operate and the time for charging up smoothing condensers associated with the oxide rectifiers. Another serious objection is that at the time of the overload, short-circuit, or fiash back, the voltage of the power source may be so far reduced that there is insufficient voltage to operate the relays and to build up sufilcient negative bias in the rectifier circuit to clear the overload, shortcircuit or fiash back. In other words, when the grid control is most needed, it may be entirely ineffective and then only the relatively slow acting sixty-cycle alternating current overload protection, or the low voltage circuit breaker releases, are available to clear a short-circuit after a relatively long time period during. which permanent injury to the rectifier and associated equipment may have taken place.

In some other systems used in the prior art, a motor generator set is provided which runs continuously to provide a negative output which is applied to the control grids of the main rectifier by means of high speed relays when it is necessary to clear a short circuit in the rectifier or its load. In such a system, the mechanical inertia of the motor generator keeps it running and effective for a long enough time to block the rectifier with negative potential even though the power voltage may have been virtually removed from the driving motor due to the short circuit.

The complexity and expense of such systems is avoided by the present invention. Herein there is obtained a more reliable and failure-proof systern by energizing the protective grid blocking equipment directly from the power leads, suppl ing the rectifier, so that the overload or shortcircuit current, itself, establishes a blocking bias necessary to block itself, and to limit the shortcircuit current. Briefly, in the simplest arrangement of my invention, current transformer; have their primary windings connected in series with the power leads. Their secondary windings are arranged to supply an auxiliary rectifier, preferably that of the cuprous oxide or selenium types, from which grid blocking potentials are obtained. Then, if a flash-back or short-circuit should occur in the rectifier or load, the current drawn automatically builds up the blocking bias across a condenser to stop the overload current without any relays Or similar mechanical devices being operated.

In order that the biasing rectifier may not be energized appreciably for rectifier currents up to normal full load, non-linear passive resistors are connected in series with the current transformer secondary. These non-linear resistors prevent much fiow of current to the biasing rectifier up to the full load current input of the main rectifier,

linear resistor automatically decreases to allow rapidly increasing current flow. In order that the biasing rectifier may be efiective on moderate overloads without being subjected to excessive inputs or severe over-loads, in case of heavy overloads or short circuits, afurther aspect of my invention includes shunting the primary or secondary of the current transformers, the biasing rectifier input and the rectifier direct current output with additional non-linear resistors to automatically limit the negative voltage delivered to the main rectifier grids and the biasing rectifier.

This invention will best be understood by referring to the accompanying drawings, in which:

Fig. l is a circuit diagram of a three-phase, full wave rectifier with the features of my invention added thereto;

Fig, 2 is a curve of the current flow of the oxide rectifier showing how it follows the variation of the main rectifier input current; and

Pig. 3 shows another type of protective circuit transformers which are well of the current the'oxide rectifier system .which has such as of the to individual rectifier tubes.

Referring now in detail to Fig, 1 of the drawings, the main power supply 8 is connected to alimliary current transformers 2, s and 6 having their primaries P connected in series with the input leads of the rectifier power transformer windings 5, t, and I. These auxiliary transformers may be so designed as to have low excitation reactance so that the voltage drop in the rm windings for normal rectifier input current is quite low. Preferably, the current transformers are equipped with cores having adjustable air gaps and with means for varying the number of t in the circuit to facilitate adjustment. As an alternative arrangement, trans= iormers may he used which have normally high excitation reactance provided the primary and/or secondary windings are shunted with an adjustable low impedance. The auxiliary transformers must have their windings insulated from one another and from ground for the voltage to which they are subjected in the circuit. Normally, the secondary windings of the auxiliary transformers must be quite highly insulated from the primary windings and in this respect will be quite like the highly insulated windings of cathode heating art and normally used with the hot cathode type of rectifiers. Across the secondary windings 8 transformers there are connected protective non-linear resistors 8, 8, it, ll, 52 and known in the prior.

i8, whichare preferably that of the typeknown in the trade as Thyrite," which possesses the property of rapidly decreasing resistance when the voltage acros the resistors is increased. For example, ordinarily, a doubling of the voltage across a piece of Thyrite material increases the current through it about 12.6 times. In this case, the Thyrite, which could be placed across the transformer primary P in some cases, protects the whole grid biasing system from excessive potentials during short-circuits or There are also six additional fThy'rite resistors it, i5, it, it, is and is placed in series with the leads from the transformer These series Thyrite" small current to fiow normal input current when the main rectifier has but allow a rapidly increasceeded. Then, when the proper selection and adjustment of the Thyrite" resistors and of the ordinary resistances which may be associated with them in the circui as shown, has been made; potential appearing across the output of comprising rectifiers 20,- ii, 22. 23, 24, 25 and therefore the biasing potential applied to the grids of the main rectifier tubes follows a curve which is shown by Fig. 2. It will be noted that in this curve there is a variamain rectifier input current. The auxiliary rectifiers'ill, 2t,- 22, 28, Ifand 25 are preferably of the copper oxide, selenium or similardry non-' vacuum rectifiersso polarized that overload currents in the there is a condenser 33 conoutput ofthe oxide rectifier: capacitance value as to hold the main rectifier grids negativefor an appreciable time aftera flash-back or short-circuit occurs, in order to allow deionization and cooling circuit elements at which the curred which caused the overload.

nected across' the There may secondary winding. resistors cause only a very 1 input to the main rectifier 2B build I up a negative blocking voltage on the control grids. Preierably.

Thyrite resistor 36 is connected across the terminals 'of condenser 33,- as shown to protect it from excessive potentials.

Although in Fig. 1, I have indicated the rectifier 28 as one utilizing high vacuum grid controlled tubes, or the Thyratron vapor or gaseous discharge type of tubes having hot cathodes 3?, 8s, as, 68, H and 62, the main rectifier may, of course, be of the steel tank mercury pool type or any other type where the dew of current can be controlled by means of controlled electrode potential. Fig. 1, at :38, indicates a phase changer type of alternating current grid excitation or control for the main rectifier, but it is also my intention that the new protective system of this invention be used in combination with new orall other types of control and protective devices according to the results desired.

For example, Fig. 3 shows two individual rectifier tubes 5i and 52, having their cathodes 58 and 54 connected to one sideof the direct current power output through series connected auxiliary transformers 55 and E6. The midpoint tap oi current input power transformer at connects through a retectance device to to the other side of the direct current power output. A condenser. 68 is placed across the direct current power output circuit. The outer ends of the secondary oi transformer 51 are connected to the anodes of rectifiers ill and 52. Transformers 5B and 56 have Thyrite" resistors 58 and 59 connected h-backs.

' any cause results in failure oction which varies in value with variations in the acrosstheir primaries; The secondary ends of transformers 55 and at are connected to rectifiers 60, 6|, 62 and 63. A Thyrite resistor 66 is connected between the midpoint of the secondcry of transformer 55 and one side of cathode as.

Rectifier tube 52 has its cathode is connected in a similar manner, that is, a Thyrite" resistor 35 is connected in series between cathode t5 and the midpoint tap on-the secondary of transformer St. A condenser as is connected in series between the anode and grid of tube at. Also. a condenser 10 is connected in series between theanode and grid of tube 52. In'the operation of the system .of Fig. 3. an overload in the direct current circuits. or excessive current through the rectifiers ti, 52 from the automatic application of a negative bias to the control grids to interrupt thecurrent now. In the'case of gas or'vepor rectifiers the interruption occurs whenthe anode currents, due-to the alternating current input. pass through zero and continue for perhaps one to several cycles of alternating current input.

The two systems dmcribed above allow the rectifiers to become operative automatically again immediately after the flash-back or short-circuit is cleared. Thus, such an-i'mproved system proexample, by the puncture of aby-pass or smoothcaused, for

ing condenser in the output circuit of the main rectifier. To take care of permanent failures or short-circuits which are more than the ordinary momentary short-circuits or flash-backs, the ordinary overload devices of the kind which are well known in the prior art to shut oil? the power completely and to sound an alarm where there is a permanent failure or short-circuit may be included. Furthermore, if desired, the operating coil of a pulsating or ratchet type relay may be connected in series with the output of the oxide rectifiers 20 to 25, inclusivashown in Fig. 1, and

in Fig. 3, rectifiers 60 to 53, inclusive, or I mayuse a similar relay associated with the power circult to shut oi the power when the grid control an alternating current source of power, a power transformer having a primary and a secondary, an auxiliary current transformer having a primarv and a secondary, the primary of said auxiliary transformer connected in series with said power transformer and said alternating current source of power, an auxiliary rectifier connected across the secondary winding of said auxiliary transformer to build up in the grid circuit of said rectifier a potential to block the current flow therethrougn, in case of overload, temporary short-circuit or flash-back in said rectifier circuit, said auxiliary rectifier being dependent for its operation solely upon potentials developed across the secondary of said current transformer, a non-linear passive resistance connected across said auxiliary transformer-secondary, and another non-linear passive resistance connected in series with the auxiliary transformer secondary winding and said auxiliary rectifier.

2. A rectifier circuit including a discharge tube having a control electrode and means responsive to overload currents in said circuit and energized only by potentials developed as a result of said overload currents for applying a blocking potential to said control electrode, said means having associated therewith a resistor characterized in that its resistance varies inversely as the current flow therethrough whereby said blocking potential increases in amplitude more rapidly than the overload currents to a predetermined maximum and thereafter are maintained substantially constant.

-3.-A protective circuit for a rectifier having an anode, a cathode and a control grid, 9. power input circuit for said rectifier, a current transformer having a primary and secondary, said primary being connected in series in said input circuit, means for rectifying potentials developed across said secondary and operated solely by said potentials and means for applying said rectified potentials to said control grid to disable said rectifier, said current transformer having a resistor characterized in that its resistance varies inversely as the current fiow therethrough connected across on of its windings whereby potentials developed across its secondary are limited to a predetermined maximum and other resistors serially connected in said means for applying rectified potentials to said control grid, said other resistors also being characterized in that their resistance varies inversely as the current flow therethrough.

4. A protective circuit for a rectifier having an a'node,'a cathode and a control grid, at power input circuit for said rectifier, a current transformer having a primary and a secondary, said primary being connected in series in said input circuit, means for rectifying potentials developed across said secondary and means for applying said rectified potentials to said control grid to disable said rectifier, said rectifying means and said applying means being responsive to and operated solely by potentials developed across said secondary, a resistor connected across one of the windings of said current transformer and a resistor serially connected in said means for applying rectified potentials to said control grid, said resistors ance varies inversely as the current fiow therethrough whereby said rectifiers applied to said .control grid increase in amplitude more rapidly than the current flow through said rectifier to a predetermined maximum and thereafter are maintained substantially constant.

CLARENCE W. HANSELL.

being characterized in that the resist- 

